Field of the Invention
This invention relates to a pressure sensor in particular for very high pressures, preferably adapted to act externally on a measuring element, the measuring element having a central cavity and being composed of two parts which are sealingly joined in order to form the cavity, and comprising sensor means for the mechanical stress condition of the measuring element when subjected to pressure.
In principle a pressure sensor consists of a spring element (measuring element) and a measurement or sensor device. Usual commercially available pressure sensors based on silicium technology can employ membranes as spring element, provided with piezo-resistive resistors as sensor means. Spring elements as membranes are unfavorable at high pressures because they are sensitive to effects related to the clamping in a substrate with a transition to materials having an unequal modulus of elasticity. The stress detected in such membranes will be a combination of compressive and tensile stresses. If the tensile stresses become sufficiently high, a break can occur. At large deformations the stresses in a membrane will not be a linear function of pressure, which results in a non-linear signal.
Pressure sensors for high pressures as indicated above, are in general previously known, and a specific example may be found in Norwegian patent application No. 94.0785. A pressure sensor according to Norwegian patent application No. 94.0785 is based upon the Bourdon effect, with a capacitive reading of the deflection. This known pressure sensor is based on tubular measuring elements, but the internal cavity or cavities is/are always disposed eccentrically, so as to obtain a bending of the element. When such bending takes place shear stresses can be generated in the joints, which in turn can lead to failure therein. A drawback with capacitive reading is that when very high pressures are concerned, it is desireable to have very small outer diameters, which results in extremely small capacitances. It is very difficult to detect these. Another drawback with capacitive detection consists therein that the measuring element can not be directly subjected to the process medium. This is because the capacity is sensitive to the dielectric constant of the medium and will be very sensitive to contamination in the form of particles which can get into the capacitor gap.
In the book "Instrumenteringsteknikk" by Ole A. Solheim, Tapir publishers Trondheim 1966, there is a description on page 118 of the measurement principle employed here. According to the book this measurement principle comprises the attachment of strain gauges on a tubular measuring element. On the surface of the element the stresses in the azimuthal direction will always be twice as high as in the axial direction. The difference in stress between these two directions will be proportional to the pressure. If there is provided for application of the highest pressure externally of the tubular element, tensile stresses that can lead to breaking are avoided. The known measurement principle is in part utilized in a more modern design according to European patent publication 0.107.549. The designs according to this publication, however, do not take advantage of the difference between axial and radial stress upon external or internal pressure application, but are only based on deformation changes. The sensor devices operate with acoustic surface waves, which means that the complete pressure sensor must have total dimensions above a certain lower limit.
Another and somewhat less interesting example from the patent literature, is to be found in French patent publication 2.531.533.